Early monsoon onset over Kerala likely

Onset of South-West monsoon over the Kerala coast may not take place until May 21, but can follow immediately thereafter, according to latest estimates.

The prognosis for the two branches — Arabian Sea and Bay of Bengal — suggests that the monsoon would ride the ongoing wet phase of the Madden-Julian Oscillation (MJO) wave to make an early onset this year.

ALTERNATING PHASES

The MJO wave travels periodically from west to east in the higher levels of atmosphere and has alternating ‘dry' and ‘wet' phases setting up rains or dry conditions as the case may be.

Though confined to the higher levels of the atmosphere, the MJO wave has a significant bearing on monsoon flow dynamics at the lower levels, variously setting up onsets, low-pressure areas and even cyclones. At the other end of the scale, the ‘dry' phase of the MJO wave is blamed for the intra-season rain shutouts and incidental ‘break monsoon' phases.

According to Dr Akhilesh Gupta, leading operational forecaster and Adviser to Department of Science and Technology, this time, the onset over Andaman and Nicobar Islands, Sri Lanka and Kerala would happen within days of each other.

As per early analysis, it is likely that the onset would happen over the Bay Islands around May 18, which would be followed close on the heels (within the next two days) over Sri Lanka.

The build-up is such that this should lead to the eagerly awaited ‘burst' over the Kerala coast almost immediately thereafter but Dr Gupta finds that the amplification of the flow pattern and moisture carry mostly confined to below the 10 deg Latitude.







Thus, the activity is seen to be concentrated over a stretch of seawaters between Arabian Sea and the Bay of Bengal (referred to as ‘Indian Sea') during this phase.

The Arabian Sea arm of monsoon may have been seen having an ‘edge' until now, but the Bay arm is catching up fast backed up by strong southerly flows to precipitate onset over the Andaman Islands in the usual time band.

The causative wet phase of the MJO wave, though, is shown by various models as exiting the Indian region even as the monsoon flows get into the groove (around May 19) and much before the onset over mainland India.

This is because, after the flows entrench themselves, the monsoon would be able to flourish on its own without the MJO prop to advance further into the mainland.

The next wet phase would not set in until June 15, according to the outlook by the Empirical Wave Propagation model employed by the Climate Prediction Centre of the US National Weather Services. A strong monsoon ‘pulse' could be expected to be triggered along with this.

Before this can happen, both Arabian Sea and Bay of Bengal are likely to witness the genesis of west-to-east oriented weather systems around May 25. According to Prof Paul Roundy, a renowned ocean weather forecaster based in the US, the Arabian Sea system may head towards the west coast.

By the same token, the Bay system, though weaker in strength, may hurtle itself towards the West Bengal and adjoining Bangladesh coasts.